The present invention relates to a golf ball material which is highly suitable for use as a cover material in golf balls, especially a golf ball material for use as an inner cover material in multi-piece solid golf balls composed of a core and a cover of two or more layers encasing the core. The invention also relates to a method of preparing such a golf ball material.
Ionic resins, or ionomers, are conventionally used in golf balls. To impart the ball with a higher rebound or a lower spin rate in particular, the desired effect is more readily achieved when the ionic resin has a higher acid content. Art for imparting the ball with a higher rebound or a lower spin rate by highly neutralizing the ionic resin has also been disclosed. Examples of such methods that are being practiced include methods which involve blending ionomers together, methods in which other thermoplastic resins and/or additives are blended together with an ionomer, and methods which increase the degree of neutralization of an ionomer itself.
Among methods that involve blending ionomers together, a number of techniques which use in combination two ionomers of differing weight-average molecular weights have been proposed. For example, U.S. Pat. No. 7,462,113 discloses the use in a cover material of a ternary ionomer having a weight-average molecular weight of from 80,000 to 500,000 in combination with a ternary ionomer having a weight-average molecular weight of from 2,000 to 30,000. Also, U.S. Pat. No. 7,273,903 describes blending together, as a cover material: a ternary ionomer having a weight-average molecular weight of from 80,000 to 500,000, a binary ionomer having a weight-average molecular weight of from 2,000 to 30,000, and an optional thermoplastic elastomer.
Golf ball materials which use ionomers and blends wherein the content of unsaturated carboxylic acid (sometimes abbreviated below as “acid content”) falls in a specific range have also been disclosed. For example, U.S. Pat. No. 6,267,693 describes an ionomer blend having an acid content of from 18.5 to 21.5%, and U.S. Pat. No. 5,873,796 describes a blend of an ionomer having an acid content of 19% with an ionomer having an acid content of 15%. Also, U.S. Pat. No. 5,222,733 describes a golf ball composition wherein at least 30% of the composition is an ionomer having an acid content of from 20 to 30%.
However, when an ionic resin having a high acid content is highly neutralized, the durability declines. For this reason, such ionic resins are not actively used in golf balls. Also, ionic resins with a high acid content often have too low a melt flow index (MI), making them difficult to injection mold, which has sometimes resulted in a poor workability when manufacturing golf balls.
In addition, art which attempts to increase the durability by using inorganic fillers and the like has been proposed. However, due to the combination of the shape of the inorganic filler and the molding method, inorganic filler-induced orientation arises, making it impossible to achieve the desired effect.